Planar microstrip structures are useful in many microwave applications because they are small, light weight, simple to fabricate, and lend themselves to use with high temperature superconductor technology. These applications include satellite communications, cellular phone communications, meteor burst communications, radar, automobile collision avoidance systems, wireless local area networks and general fixed frequency radio communications. Recently, microstrip patch structures have been the focus of much interest due to there use in filters, as patch antennas, and as resonators, all of which are used in the above listed applications.
Planar filters can be built in either single mode or dual mode configurations. In the single mode configuration each metalized patch is a single resonator for the filter. In the dual mode configuration, each dual mode patch acts as two coupled resonators in the filter.
Dual mode elliptic filters are routinely used in satellite communications, where low loss, high Q performance is required. This type of filter is also used in multiplexer applications, since the sharp filter skirts mean that crosstalk between the frequency bands can be kept to a minimum. Due to their high frequency selectivity, dual mode elliptic filters are preferred for both space borne, and ground based applications. These filters have been constructed for microwave applications using several techniques, including cylindrical and rectangular cavities, dielectric resonators in cavities, as well as planar microstrip. Of these, the planar microstrip configuration results in smaller, lighter weight structures than are possible with the other technologies. Compactness and light weight are crucial factors for any filter used on a space platform. The planar structure is also well suited for use with high temperature superconductors, and therefore can be the basis for an extremely low loss filter, with greater power handling capability.
Considerable empirical work has been done with dual mode microstrip patches and coupling between these patches and microstrip lines. Useful elliptic function planar filters have been designed and built, but the lack of analysis has made these filters larger than necessary and has meant that many design iterations were required. Computer simulation allows the construction of dual mode filters with a minimum number of design iterations and tuning steps. Methods for accurate electromagnetic computer simulation useful for efficient filter design are disclosed in the dissertation entitled, Accurate Analysis and Computer Aided Design of Microstrip Dual Mode Resonators and Filters, Preston Whitfield Grounds, III, (1995), hereby incorporated by reference.